1. Field of the Invention
The present invention relates to the field of power electronics. It proceeds from a power semiconductor component having pressure contact plungers according to the preamble of claim 1.
2. Discussion of Background
Power semiconductors for industrial, tractive and other applications frequently include power semiconductor components such as, for example, thyristors, GTOs (gate turn-off thyristors), MCTs (MOS-controlled thyristors), power diodes, IGBTs (insulated gate bipolar transistors) or MOSFETs (MOS-controlled field effect transistors), which are encapsulated in a housing having pressure contacts. Such pressure housings can include, for example, a large-area thyristor or a plurality of small-area MOS-controlled semiconductor chips.
Such a power semiconductor component is disclosed, for example, in U.S. Pat. No. 4,500,907. The typical design comprises a semiconductor wafer with metallized power contacts, contact plates bearing on both sides and two cylindrical metal plungers by means of which the arrangement is held together in a housing. The contact plates consist of molybdenum (Mo) or tungsten (W) having a coefficient of expansion matched to silicon. The metal plungers serve the purpose of making electric and thermal pressure contact with the power semiconductor and of dissipating heat to a heat sink. The required high contact pressures lead to the known problem that damaging excess pressures can occur in the semiconductor wafer at the edge of its contact surface with the rigid contact plate. It is proposed as a solution to mill in a groove, for example a rectangular, V-shaped or arcuate one, on the circumference of the metal plunger, in order in conjunction with homogeneous pressure loading from outside to achieve pressure relief at the edge side by elastic deformation of the groove.
Proposed in U.S. Pat. No. 5,539,220 is a flat pressure housing for several semiconductor elements arranged next to one another, in which the contacting and heat dissipation are provided by pressing on large-area, common electrode plates. Such a pressure contact housing is preferably fitted with several IGBT chips and a free-wheeling diode. Advantages of said design relate to improving the heat dissipation and eliminating the bond wires and their inductances. Thermoplastic or solder layers between the electrode plates and semiconductors, and matching the overall height of IGBTs and free-wheeling diodes are disclosed as measures for ensuring homogeneous contact pressure over all structural elements.
Further reaching problems owing to inhomogeneities in the contact pressure impressed from outside onto the components remain unsolved in both publications. This relates first and foremost to unevennesses in contact plates and nonuniformly acting clamping devices. If a contact plate or a clamping device is deformed, or should they become deformed in the course of time owing to a change in load, material fatigue or the like, the result is a nonuniform distribution of the contact pressure over individual or several power semiconductor surfaces. This can lead to local overheating and to the failure of a semiconductor element. In the case of GTOs, for example, one of the most frequent causes of failure is the deformation acquired by the contact plates and metal plungers during operation. Thus, 10 mm thick contact plates made from copper can have unevennesses of 100 .mu.m or more. Slight deformations of the contact plates already occur, however, when they are being tested before installation.